1. Field of the Invention
The present invention relates to electron guns that can be used to supply a modulated electron beam in an electron tube oscillator or amplifier, or in particle beam accelerator injectors, or the like.
2. Prior Art
There is known from the prior art, through articles and U.S. Pat. No. 4,313,072, electron tubes termed "lasertrons" that employ an electron gun in which the electron beam is modulated by laser pulses illuminating a photocathode. The present invention brings improvements to this technology by obviating the need for a laser.
In prior art lasertrons, a photocathode is illuminated by a laser beam whose wavelength is chosen in terms of the work function of the material constituting the photocathode. Accordingly, a laser beam pulsed at a frequency F knocks out electron packets from the photocathode at the same frequency F. These electron packets are then accelerated by an electrostatic field and thus acquire kinetic energy. They then cross a cavity resonant at frequency F and their kinetic energy is transformed into electromagnetic energy of frequency F. The energy is drawn off from the cavity by coupling it to an external user circuit.
In FIGS. 1 and 2, two embodiments of prior art lasertrons are represented schematically in longitudinal cross-sectional views.
In these figures, numeral 1 designates the photocathode, numeral 2 the laser beam, and numeral 3 the electron beam.
In the embodiment of FIG. 1, the photocathode is illuminated at an oblique angle by a laser beam 2 and the electron beam 3 propagates along the tube's longitudinal axis XX'.
In the embodiment of FIG. 2, the laser beam 2 and the electron beam 3 propagate along the tube's longitudinal axis XX', but in the opposite direction.
The laser beam 2 is thus normal to the photocathode's emissive surface.
The electron beam 3 is accelerated by the electrostatic electric field created by the anode 4, and then enters a cavity 5 resonant at frequency F. The electron beam is then received by a collector 6. The electromagnetic energy at frequency F is drawn off at the cavity 5 by coupling the latter to an external user circuit, using a waveguide 7 associated to a window 8, as shown in FIG. 1, or by a loop, as shown in FIG. 2.
The interest in having electron guns modulated in this manner is that it allows the tubes to be made very compact.
In lasertrons, electron packets are knocked out of the photocathode at frequency F; these electrons are thus naturally grouped from the outset, whereas in tubes such as klystrons several cavities are required to form packets of electrons from an initially continuous electron beam. The electron packets can also be injected into particle accelerators operating at frequency F.
The drawback with electron guns pulsed in this manner is that they are limited in frequency and in power.
For instance, to produce high powers, a large current needs to be drawn, which calls for a photocathode having a large surface and involves passing a sizeable beam into the lasertron cavity. This in turn requires that the lasertron cavity dimensions be sufficiently large to accommodate the passing electron beam, which limits the operating frequency. Moreover, the use of a large-size cavity produces a poor coupling between the beam and the cavity, which adversely affects efficiency.
Furthermore, the maximum obtainable modulation frequency for an electron gun modulated by illuminating pulses form a laser is limited by pulsed laser technology.
The electron guns embodied in FIGS. 1 and 2 have the following drawbacks regarding the use of a laser illuminating source:
the photocathode's photoelectric efficiency is not optimal at the wavelengths currently supplied by lasers; PA1 the modulation frequency F is limited by the state of the art of laser pulse modulation; PA1 to overcome the above-mentioned drawbacks in the prior art, ancillary devices are added to the system to obtain a better adapted wavelength and to control the laser modulation as best as possible.
the bulk, weight, complexity and cost of the pulse-modulated illuminating system are inconvenient for practical applications.
In theory, lasertrons could develop very high RF energy levels with excellent efficiency (several megawatts peak power with an efficiency on the order of 70%, i.e. twice or one and a half that obtainable with a pulsed klystron). However, in the state of the art, there remain technical problems with existing devices.
The technology involving a gun excited by a laser essentially rests on the cathode and the laser. The progress in photocathode (GaAs. . . field emission cathodes) is encouraging if still insufficient. Several tens of amps are easily obtainable in laboratory conditions ;however the objective is on the order of a kA for a period of 50 to 100 picoseconds. As for the laser, which is located outside the tube itself, there still remains a number of basic difficulties to resolve before a making judgement. The invention covered by the present patent application proposes to replace the laser by a much simpler source.
Lasers, which can e.g. be YAG lasers, have low efficiencies and their setting up conditions are critical. The excitation of the photocathode requires very short wavelengths, e.g. in the ultraviolet (UV) region in order to have a good photo-electron conversion efficiency. Since laser emission wavelengths are generally greater than desired, a light frequency multiplying device is added to the system. Such multipliers work satisfactorily, but further complicate the system, which then becomes even more critical. Moreover, efficiency is further diminished.
But an even greater drawback lies in that it is extremely difficult to modulate lasers in microwave pulses. The present problem confronting laser manufacturers is the impossibility of producing the signals necessary for properly operating the lasertron, and shown in FIG. 3. In each micropulse of width the micropulse frequency corresponds to that of the laser or the frequency multiplier; the frequency 1/T of these micropulses is the microwave frequency, several GHz, which the "lasertron" tube is to amplify. The best prototypes are limited to several tens of micropulses at approximately 250 MHz with variable intensity (FIG. 4).